This invention relates generally to distance measuring apparatus, and more specifically, to a combined target and prism housing assembly for use in conjunction with distance measuring instruments that employ infrared or laser ranging systems.
The last decade or so has seen the rapid development of microprocessors and infrared and laser optical systems with the resulting development of various electronic distance measuring systems for use in civil engineering applications including surveying. Typically, such electronic distance measuring systems are used to provide range and angular information with respect to remotely located reflecting devices placed at ranges up to 10 or more kilometers from the measuring instrument. Well-known manufacturers of such laser or infrared electronic distance measuring instruments include Hewlett-Packard, the Lietz Company, and A.G.A. Corporation. Usually such electronic distance measuring instruments consist of two main components, namely, the electronic ranging system that utilizes a transmitter/receiver of either infrared or laser energy that is transmitted to a remotely and previously placed reflector assembly, and a sighting telescope that is used to accurately point the ranging system transmitter at the distantly located reflector. Consequently, the remotely located reflectors which utilize retro-reflector devices such as prisms or other such reflecting optics that reflect incident energy on a path that is colinear with that incident energy, are used with a targeting device that permits the surveyor or other user of the equipment to accurately align the ranging electronics and the target by sighting through the telescope to align cross hairs with the remotely positioned target center point.
Even though retro-reflector devices or prisms are designed to return incident infrared or laser energy over a broad range of incoming angles with respect to the face of the reflector, the sensitivity of the receiver portion of the electronic distance measuring instruments usually requires that the reflector be adjusted to be substantially normal to the incoming energy. Consequently, the remotely located target prism combination should be adjustable in azimuth and elevation to permit the user to adjust the reflector face until it is substantially normal to the incoming energy to maximize the level of reflected infrared or laser energy available to the receiver.
The indicated developments in optics and electronics, render the aforementioned electronic distance measuring devices accurate to within a few centimeters or less. Therefore, it is desirable that adjustment of the reflector relative to the targets used for sighting of the telescope, produce no significant error that would be substantially equivalent to or greater than the intrinsic accuracy of the optics and electronics. Otherwise, one of the principal advantages of such new and costly instruments would be substantially defeated. Accordingly, it is important that the distance between the center point of the reflector, or reflector combination, and the sighting target remotely located from the measuring instrument, be, within a very small tolerance, equivalent to the distance between the longitudinal axis of the telescope and the ranging axis over which infrared or laser energy is transmitted and received by the measuring instrument. In addition, it is important that any adjustment of the reflectors to obtain the normalizing relationship referred to above, not incur any additional error between the target point and the reflection point at the remotely located reflection device. To preserve the accuracy noted above, manufacturers of electronic distance measuring instruments also make available various prior art combined reflector and target devices with a substantially fixed offset distance between the target and reflector and a means of adjusting the target in azimuth and elevation. These prior art devices are designed for compatibility with that particular instrument with which it is intended for use and without introducing any substantial additional errors.
Unfortunately, each such prior art target and reflecting device is thus limited for use with one type of reflector such as only a unitary round retro-reflector prism, or only a lateral retro-prism and is usually limited to the above-indicated offset distance between the target and reflecting objects with which the electronic distance measuring instruments is designed to operate. As a result, users of such systems are constrained to use only a specific manufacturer's reflector and target device in conjunction with that manufacturer' electronic distance measuring instrument. This constraint can at times be highly disadvantageous. For example, this disadvantage becomes difficult when one surveying team uses electronic distance measuring systems manufactured by different companies, each employing non-standard offsets or when the differences in instrument sensitivities or in ranges to be measured require different numbers of prisms to reflect sufficient energy back to the devices receiver.
In addition to the above-noted disadvantage, many prior art reflector-target combination devices are not designed to provide adjustment in elevation and azimuth without incurring substantial error in range or angle in comparison to the accuracy capabilities of the measurement instruments with which they are used.